Developmental Patterns of Doublecortin Expression and White Matter Neuron Density in the Postnatal Primate Prefrontal Cortex and Schizophrenia

Postnatal neurogenesis occurs in the subventricular zone and dentate gyrus, and evidence suggests that new neurons may be present in additional regions of the mature primate brain, including the prefrontal cortex (PFC). Addition of new neurons to the PFC implies local generation of neurons or migration from areas such as the subventricular zone. We examined the putative contribution of new, migrating neurons to postnatal cortical development by determining the density of neurons in white matter subjacent to the cortex and measuring expression of doublecortin (DCX), a microtubule-associated protein involved in neuronal migration, in humans and rhesus macaques. We found a striking decline in DCX expression (human and macaque) and density of white matter neurons (humans) during infancy, consistent with the arrival of new neurons in the early postnatal cortex. Considering the expansion of the brain during this time, the decline in white matter neuron density does not necessarily indicate reduced total numbers of white matter neurons in early postnatal life. Furthermore, numerous cells in the white matter and deep grey matter were positive for the migration-associated glycoprotein polysialiated-neuronal cell adhesion molecule and GAD65/67, suggesting that immature migrating neurons in the adult may be GABAergic. We also examined DCX mRNA in the PFC of adult schizophrenia patients (n = 37) and matched controls (n = 37) and did not find any difference in DCX mRNA expression. However, we report a negative correlation between DCX mRNA expression and white matter neuron density in adult schizophrenia patients, in contrast to a positive correlation in human development where DCX mRNA and white matter neuron density are higher earlier in life. Accumulation of neurons in the white matter in schizophrenia would be congruent with a negative correlation between DCX mRNA and white matter neuron density and support the hypothesis of a migration deficit in schizophrenia

Candidate genes in schizophrenia

Schizophrenia is a common and devastating neuropsychiatric disorder, that affects approximately 1% of the human population. Various family, twin and adoption studies indicate genetic factors are of major aetiological importance. For the past 20 years investigators have been concentrating their efforts on dopamine, because this neurotransmitter and it's receptors (specifically DRD2, DRD3 and DRD4) have been implicated in the disease. In addition, recent findings of trinucleotide repeat expansions that have been shown to cause other neurological disorders, as well as the emerging evidence of linkage, have rekindled hopes to find the genetic contributions to schizophrenia. The research described in this thesis has explored these three aspects of this disease (dopamine hypothesis, expansion of trinucleotide repeats and linkage) in two population samples (from East Anglia and East Yorkshire) of unrelated subjects (156 in total) and matched unrelated controls (170 in total). Examining the relationship between the dopamine D2-like receptors and schizophrenia have excluded the DRD4, a 500bp 5'-upstream region of the DRD4 and a 600bp 5'-upstream region of the DRD2 as plausible regions harbouring mutations that could predispose to schizophrenia in the two population samples investigated. However interesting results were obtained for the DRD3. A 2Kb stretch of the previously uncharacterised 5'-upstream region of the human DRD3 was isolated, sequenced and partially characterised. A 1Kb region of this putative promoter was screened for polymorphisms in all case and control subjects from the two population samples by the single stranded conformation polymorphism method. Four polymorphisms were identified and found to be in linkage disequilibrium. The genotypes of these four sites did not show association with disease. However, when the Msc1 polymorphism of the DRD3 (one that showed no association with disease when investigated in the two population samples) was included in the genotypes, a significant association with disease was obtained.Expansions of genomic CAG triplets were found to be associated with schizophrenia and early age at onset in the East Anglia population sample. To identify the loci containing this expansion, the length of the CAG repeat of 108 loci known to contain CAG repeats was examined in a selected number of patients. All 108 loci were excluded as potential candidates. Using methodologies suited to non-Mendelian segregation, several groups have reported significant evidence for a schizophrenia susceptibility locus within a 30cM region within chromosome 6p24-22. A 20cM region in which suggestive lod scores were previously found, was examined further with 9 microsatellite markers. The frequency of alleles were measured in cases and controls for each marker, in a case/control association study design. Two tightly linked markers, D6S429 and D6S338 were found to be associated with disease. This implies that either this 0.04cM interval is in linkage disequilibrium with a susceptibility locus or carries a susceptibility locus within this interval. Therefore this finding corroborates the prior evidence for a susceptibility locus within this interval and appears to refine the region